1. Field of Invention
The invention relates to the tunneling magnetoresistance device and, in particular, to the tunneling magnetoresistance device possessing a high magnetoimpedance (MI) effect.
2. Related Art
Magnetic random access memory (MRAM) and magnetic detector have considered as the most potential and applicable devices with high magnetoresistance ratio due to the peculiar interface characteristic between the magnetic layers, indicating the phenomenon of high resistance variations (ΔR/R) have increased record density of commercial hard disk up to 150 Gb/in2, and made the read-out ability of tunneling magnetoresistance device higher than that of the spin valve. Thus, the spotlights of spin research were focused on how to improve on the ΔR/R variation and find total solutions by adjusting the different composition and arrangement between stacking magnetic multi-layers. For instance, an anti-ferromagnetic coupled (AFC) inserting layer is used to increase the ΔR/R variation for 30 to 40%. Further study about sensitivity and reproduction characteristic is another important topic in the field of spin electronic devices. For this reason, most researchers are trying to improve on the higher MR ratio efficiently. These methods were used to try changing the film structure of the intermediate insulation layer, and looking for the semi-mental material as magnetic inserting layer that has near 100% spin-polarization (SP). However, highest MR ratios were presently obtained only about 30˜40% and the reproduction of structure and characteristic are not good enough.
For instance, U.S. Pat. No. 6,657,830 discloses a spin valve structure that use stacking magnetic layers of different composition or semi-metal materials with high spin polarization to compose soft ferromagnetic layers and ferromagnetic layers. The CPP measurements were carried out at normal temperature with bias voltage of ±0.2V. As shown in FIG. 1, the drawing shows a highly asymmetric MR curve and a little smaller ΔR/R ratio with applied positive bias voltage. This method improves poor stability of the conventional MR detector. With reference to FIG. 2, U.S. Pat. No. 6,219,274 discloses three-layer structures that are composed of a first ferromagnetic layer 1, a tunnel barrier layer 3, and a second ferromagnetic layer 5. According to the U.S. Pat. No. 6,219,274, a radical oxidation process was used to make MR devices high reproduction and stability. However, only 13% of ΔR/R variation can be obtained after adjusting optimal processes. Please refer to FIG. 3. Japan patent JP2002-025017 discloses a structure that inserts an MR device between an upper shield electrode 7 and a lower shield electrode 9. The patent also studies signal to noise (S/N) ratio with relations to driving frequency of current and R×A ratio, but this patent doesn't disclose the detailed relationship between frequency of alternating current and resistance/impedance of the MR device. Further, this patent also doesn't mention in depth about real component (MRe) and imaginary component (MIm) of magnetoimpedance after complex number conversions. In another Japan patent JP H09-198622, a ferrimagnetic layer is used to replace one ferromagnetic layer of the three-layer structure and study the MR property. However, this patent only provides B-H relation drawing, and no figures about the MR curve.
According to the descriptions above, they all mention how to use MR characteristics for the read-out device but the best value of the MR value was obtained only about 30%˜40%. Therefore, it is necessary to research further more about how to improve the MR value.